1. Field of the Invention
This invention relates to an improved crash attenuator for protecting a vehicle from impacting a rigid backup. More specifically, this invention relates to an improved crash attenuator that redirects or gates and redirects vehicles.
2. Description of the Related Art
Impact attenuation devices are often used to prevent the vehicles from impacting a rigid backup. A rigid backup may be any relatively inflexible item, stationary or portable, that would be undesirable to impact. An example of a rigid backup is a toll booth median between two lanes of traffic.
Impact attenuation devices perform gating functions, redirecting functions or both. The gating function absorbs impact energy through a vehicle penetrating the gating device or portion of the attenuator. Vehicles traveling toward the rigid backup in the axial direction of the attenuator impact the gating device or portion and slow down through the length of the device. However, vehicles approaching the "coffin corner" of the rigid backup from an angle off of the axial direction of the attenuator do not have full length of the gating device or portion to absorb impact energy. The coffin corners are the front corners of the rigid backup. Without the impact energy being absorbed, the full force of the vehicle impacts the coffin corner, resulting in catastrophic damage.
To inhibit vehicles impacting the coffin corner of a rigid backup, a redirecting device or portion redirects the vehicle away from the coffin corner. To accomplish this, the redirecting device or portion must be designed to withstand lateral impact.
One approach to such impact attenuation devices employ an axially collapsible frame having compression resistant elements disposed one behind the other in the frame. Young U.S. Pat. No. 3,674,115 provides an early example of one such system. This system includes a frame made up of an axially oriented array of segments, each having a diaphragm extending transverse to the axial direction and a pair of side panels positioned to extend rearwardly from the diaphragm. Energy absorbing elements (in this example water filled flexible cylindrical elements) are mounted between the diaphragms. During an axial impact the diaphragms deform the energy absorbing elements, thereby causing water to be accelerated to absorb the kinetic energy of the impacting vehicle. Axially oriented cables are positioned on each side of the diaphragms to maintain the diaphragms in axial alignment during an impact.
Other examples of such crash barriers are shown in Walker U.S. Pat. No. 3,944,187 and Walker U.S. Pat. No. 3,982,734. These systems also include a collapsible frame made up of an axially oriented array of diaphragms with side panels mounted to the diaphragms to slide over one another during an axial collapse. The barriers of these patents use a cast or molded body of vermiculite or similar material or alternately loosely associated vermiculite particles to perform the energy absorption function. Obliquely oriented cables are provided between the diaphragms and ground anchors to maintain the diaphragms in axial alignment during a lateral impact.
Gertz U.S. Pat. No. 4,352,484 discloses an improved crash barrier that utilizes an energy absorbing cartridge made up of foam filled hexagonal lattices arranged to shear into one another in response to the compression forces applied to the energy absorbing cartridge by an impacting vehicle.
Stevens U.S. Pat. No. 4,452,431 shows yet another collapsible crash barrier employing diaphragms and side panels generally similar to those described above. This system also uses axially oriented cables to maintain the diaphragms in axial alignment, as well as breakaway cables secured between the front diaphragm and the ground anchor. These breakaway cables are provided with shear pins designed to fail during an axial impact to allow the frame to collapse. The disclosed crash barrier is used with various types of liquid containing and dry energy absorbing elements.
VanSchie U.S. Pat. No. 4,399,980 discloses another similar crash barrier which employs cylindrical tubes oriented axially between adjacent diaphragms. The energy required to deform these tubes during an axial collapse provides a force tending to decelerate the impacting vehicle. Cross-braces are used to stiffen the frame against lateral impacts, and a guide is provided for the front of the frame to prevent the front of the frame from moving laterally when the frame is struck in a glancing impact by an impacting vehicle.
All of these prior art systems are designed to absorb the kinetic energy of the impacting vehicle by compressively deforming an energy absorbing structure. Because of the potential instability of compressive deformation, these systems use structural members to resist side forces that develop from compression loading. Furthermore, all use sliding side panels designed to telescope past one another during an impact. Because such sliding side panels must slide past one another during an axial impact, they have a limited strength in compression. This can be a disadvantage in some applications.
Another prior art system known as the Dragnet System places a net or other restraining structure transversely across a roadway to be blocked. The two ends of the net are connected to respective metal ribbons, and these metal ribbons pass through rollers that bend the ribbons as they pay out through the rollers during a vehicle impact. The energy required to deform these ribbons results in a kinetic energy dissipating force which decelerates the impacting vehicle. The general principle of operation of the metal deforming rollers is shown for example in Jackson U.S. Pat. Nos. 3,211,620 and 3,377,044 as well as Vanzelm U.S. Pat. No. 3,307,832. The Dragnet System utilizes the metal ribbons in tension, but it is not well suited for use alongside a roadway because metal bending systems are positioned on both sides of the roadway, and the net or other obstruction extends completely across the roadway.
Krage U.S. Pat. No. 4,784,515 describes a collapsible guard rail end terminal that utilizes a wire cable extending through grommets in legs of the end terminal. The side panels of the end terminal are mounted to slide over one another when struck axially. When the end terminal collapses during an impact, the legs may be rotated such that the grommets work the cable and create a frictional force on the cable. However, the magnitude of the resulting retarding forces is highly variable, due to the variable and unpredictable rotational positions of the legs during the collapse.
An Advanced Dynamic Impact Extension Module (ADIEM)--11 of Syros, Inc. provides a system with both gating and redirecting portions. An initial gating section comprises a row of lightweight crushable concrete modules that are placed on a ramp increasing in height toward a rigid backup. The gating portion of the attenuator is the row of modules. A vehicle impacting the modules has the impact force absorbed as the modules break apart. Vehicles approaching more from the side are redirected by the ramp. While the ramp prevents the vehicle from impact the coffin corner, the redirecting ramp is very unforgiving in that it does not absorb energy.
Thus, a need exists for a simple, inexpensive attenuation system that absorbs energy as it redirects vehicles away from the coffin corners. A need also exists for a simple, inexpensive system that performs both gating and redirecting functions.